Process for thermal production of magnesium

ABSTRACT

The invention relates to processes for production of magnesium of greater purity by reaction of a reducing agent with a substance containing magnesium oxide in the presence of a molten slag. The process according to the invention consists of agglomerating the reducing agent (ferro-silicon) with a binder formed of a mixture of water and slag which has been slowly cooled, drying and calcining and introducing the agglomerated material into the furnace in lieu of the lumps of reducing agent normally employed.

United States Patent [1 1 [111 3,837,843

Pons et a1. Sept. 24, 1974 1 PROCESS FOR THERMAL PRODUCTION 3,114,627 12/1963 Aoyama 75/67 ()F MAGNESIUM 3,441,402 4/1969 Magee et al. 75/67 Inventors: Rene Pons, Grenoble; Marcel Poyet,

. Gourdan-Polignan; Jean Desbert,

Barbazan; Andre Mena, Saint-Benat, all of France Societe Francais dElectrometallurgie, Paris, France Filed: Oct. 29, 1973 Appl. No.2 410,965

Assignee:

Foreign Application Priority Data Oct. 30, 1972 France 72.38394 us. Cl. .IF75/67, 75/3 Int. Cl C22b 45/00 Field of Search 75/67 R, 67 A, 10 A, 24,

References Cited UNITED STATES PATENTS 11/1945 Wagner et al. 75/67 FOREIGN PATENTS OR APPLICATIONS 569,744 6/1945 Great Britain 75/67 Primary ExaminerL. Dewayne Rutledge Assistant ExaminerM. J. Andrews ABSTRACT 9 Claims, No Drawings PROCESS FOR THERMAL PRODUCTION OF MAGNESIUM The invention relates to processes for production of magnesium by reaction of a reducing agent with a substance containing magnesium oxide in the presence of a molten slag.

French Pat. Nos. 987,046 and 1,194,556 describe that magnesium can be produced by reacting reducing agents, such as silicon, with a substance containing magnesium oxide in the presence of a slag, whereby lumps of reducing agent, for instance iron and silicon, and lumps of calcined dolomite, are introduced above a molten slag. The magnesium oxide dissolves in the slag, enters into contact with the lumps of reducing agent and reacts therewith, causing a strong evolution of magnesium in the vapor state.

Industrial experience over a period of several years has shown that the state of division of the ferro-silicon has a significant effect on the reaction process. Large particles tend to sink through the slag and dissolve in the underlying molten metal before their silicon content is exhausted. The action of the reducing agent becomes slow and incomplete due to the reduced area of contact between the two liquid phases, the lower concentration of silicon, and the unfavorable hydrostatic pressure.

In contrast, reducing agent which is too finely divided remains on the surface of the slag as a result of the violent evolution of magnesium vapor. Intermittent introduction of finely divided reducing agent causes sudden variations in the rate of evolution of magnesium, which may then exceed the cooling capacity of the condenser and hence lead to undesirable deposition at other points of the installation.

French Pat. No. 1,052,513 and additional application No. 63,774 describe processes enabling such irregular deposition to be avoided by suitable design of the condenser but at increased cost. Additionally, very finely divided material is subject to entrainment by the magnesium vapor to lower the purity of the metal produced.

It is impractical from a cost standpoint to employ a reducing agent of controlled narrow particle size range so that the state of division of the material employed will vary according to circumstances.

It has also been proposed, in relation to processes for producing magnesium whereby reaction takes place mainly in the solid state, to press out briquettes from a dry mixture of reducing agent and a substance containing magnesium oxide, both finely ground, or by agglomerating these materials by means of a binder, which may be water (French Pat. No. 1,733,294 and additional application Nos. 44,064, 865,136 and 983,970, US. Pat. No. 2,74l,552). The intent of preparing such briquettes is to insure improved contact between the materials which are to be reacted in the solid state.

The main disadvantage of this method, as explained in French Pat. No. 1,153,569, is that grinding dolomite or calcined magnesite in contact with air leads to pickup of moisture, which adversely affects the process in that the steam released in the reduction furnace reoxidizes magnesium vapor, thus reducing the total rate of production. The finely divided particles of magnesium oxide so formed are transmitted to the condenser where they reduce the purity of the metal deposited.

The binders normally used to bring about agglomeration also have the effect of introducing impurities into the furnace and hence into the metal and of modifying the composition of the slag, which may affect the process.

Thus, the method consisting of agglomerating the re-' action mixture beforehand with a view to overcoming the difficulties described above obviously is impractical.

Additionally, the method of production of magnesium by reaction of silicon with calcined dolomite exhibits another feature, i.e. the slag obtained, consisting of a calcium silico-aluminate plus a little magnesia, has little market value. Part of the slag is nevertheless used by the cement industry in combination with other materials to make up cements, but, in order to render it marketable, the slag has to be quenched rapidly by pouring the molten material into cold water. This obviates setting during transportation. A substantial fraction of the slag escapes this treatment and solidifies naturally in air, particularly at the bottom of the receptacle from which the molten slag is poured into the water. The non-quenched fraction disintegrates during the cooling stage of between 400 and 600 C, forming a finely divided dust for which no use has yet been discovered and which gives rise to fairly difficult problems of storage.

It is an object of this invention to overcome or substantially reduce the problems described above. In accordance with the practice of this invention, the reducing agent is introduced in the form of uniform size granules which exhibit little tendency to sink into the slag. At the same time, the reducing agent is diluted by mixing with part of the slag formed as a by-product of the reaction. Additionally, the process permits the use of a large proportion, if not all, of the non-quenched slag, which has hitherto been unusable and which has constituted a nuisance factor.

In the process of the invention, the particles of reducing agent are agglomerated by means of a binder formed of slag obtained from a previous production stage, to which water has been added. The formed agglomerates having particles within the range of 5 to 40 mm are then dried and calcined at a temperauture of not less than 700 C to remove the last traces of water of crystallization which could adversely affect operations in the furnace as previously described. The calcined agglomerates do not pick up the moisture, that is the opposite of magnesium oxide-containing agglomerates. The agglomerates calcined, as described, are then introduced into the furnace in lieu of the reducing agents employed according to the prior art.

Preferably, non-quenched slag is employed since it has been observed that, in addition to the desirability of making use of this material, which has little market value and poses storage problems, non-quenched slag gives agglomerates which exhibit mechanical properties superior to those obtained with quenched slag.

The following composition is typical of the slag obtained from the furnace:

SiO, 15-35% by weight ALgog 525% by weight CaO 40-70% by weight MgO 0-l0% by weight The above composition may be modified by additions, either before or immediately after tapping the furnace, with a view to improving the binder properties of the slag or facilitate the reaction, e.g., by increasing the magnesia content of the slag.

The agglomerates may be calcined by heating for 30 to 90 minutes at temperatures of 650 to 800 C. Surprisingly, it has been observed that silicon is not oxidized to any substantial extent by this treatment where it is surrounded by the agglomerated material, whereas the degree of oxidation is appreciable when silicon is heated alone at such temperatures.

Preferably, calcination can be carried out at a higher temperature, i.e., 900 to l,000 C, for to 15 minutes in a rotary furnace containing alumina and/or bauxite fines. It is then observed that an alumina-rich protective coating, which is formed on the agglomerates, effectively protects the reducing agent against oxidation despite the high temperature involved. The mechanical strength of the agglomerates is also increased. Furthermore, this preferred method improves the productivity of the process by reducing calcination time.

The composition of the slag (in parts by weight) was:

SiO

25% A1 0, 15% C210 55% MgO 5% Regularly shaped spherical granules were obtained by using a spherodizing machine.

This apparatus consisted essentially of a flanged disc inclined at an angle of 3045 to the horizontal, and rotated at a speed of 18 to 24 rpm. The mixture of ferro-silicon and slag is poured evenly onto the: disc, accompanied by a spray of water. The paste so formed is gradually converted into spherical granules by the rotation of the disc which gradually overflow the rim. By varying the speed and inclination of the disc, it was found possible to obtain spherical granules to 20 mm in diameter, all of which gave good results when subsequently used in the process.

Once the slag had set, the balls were calcined for 50 minutes at 700 C in a rotary furnace.

The silicon content of the balls was determined as follows: The material was finely ground, treated with concentrated caustic soda. The hydrogen evolved was passed over a drying agent and determined by measuring the quantity of water fonned by reaction with copper oxide heated to 400 C. The results of the analysis showed that oxidation of silicon during calcination was negligible.

The balls were introduced into a furnace in lieu of the 2-20 mm ferro-silicon normally employed. The furnace was of the type normally used for the thermal production of magnesium. Pressure in the furnace was found to remain very steady throughout the test, varying less than 5 mm of mercury from the mean pressure of 35 mm of mercury. The concentration of impurities in the metal obtained was reduced by 15 percent. More particularly, the silicon content was found to be 40 percent less than that of a control sample of metal produced according to the prior art, in which use was made of the same raw materials.

A further test was carried out wherein the balls were prepared as described above, but calcined in admixture with bauxite powder. Ten parts by weight of bauxite powder were used per parts by weight of balls. The furnace temperature employed was 950 C and residence time was 10 minutes. The balls so obtained were covered as previously indicated by a yellowish coating of alumina.

Analysis showed that the ferro-silicon had undergone virtually no oxidation.

These balls were tested under conditions identical to those used in the previous test and gave substantially the same results.

It will be understood that changes can be made in the details for formulation and operation, without departing from the spirit of the invention, especially as defined in the following claims.

We claim:

1. In a process for the production of magnesium wherein a reducing agent is reacted with a substance containing magnesium oxide in the presence of molten slag, the improvement wherein the reducing agent is introduced in the form of agglomerates produced by mixing the finely divided reducing agent with a binder consisting of water and slag, drying the agglomerates, and then calcining the dried agglomerates at a temperature not less than 700 C.

2. A process as claimed in claim 1 whereby the agglomerates are calcined in a rotary furnace in the presence of alumina and/or bauxite fines.

3. A process as claimed in claim 1 in which the slag used in the binder is of a composition corresponding to the composition of the slag resulting from the production of magnesium.

4. A process as claimed in claim 1 in which the slag used in the binder is from a prior production of the magnesium.

5. A process as claimed in claim 4 whereby the slag used in the binder has been allowed to cool naturally in air from the pouring temperature.

6. A process as claimed in claim 4 in which the slag used in the binder is in finely divided form.

7. A process as claimed in claim 1 in which the agglomerates are dimensioned to be within the range of 5-40 mm.

8. A process as claimed in claim 7 in which the agglomerates are calcined by heating to a temperature within the range of 650-800 C for from 3090 minutes.

9. A process as claimed in claim 7 in which the agglomerates are calcined at a temperature within the range of 900l,000 C for from 5-15 minutes.

axes

UNITED STATES PATENT ()FFICE CERTIFICATE OF CORRECTION Patent No. 837 I843 Dated pf mb r 4, 19 74 Inventor(s) Rene Pons 61; a1.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Change the name "Jean Desbert" to "Jean Desbret" Signed and sealed this 4th day of February 1975.

(SEAL) Attest:

McCOY M. GIBSON JR. c. MARSHAL DANN Attesting Officer c missione r of Patents 

2. A process as claimed in claim 1 whereby the agglomerates are calcined in a rotary furnace in the presence of alumina and/or bauxite fines.
 3. A process as claimed in claim 1 in which the slag used in the binder is of a composition corresponding to the composition of the slag resulting from the production of magnesium.
 4. A process as claimed in claim 1 in which the slag used in the binder is from a prior production of the magnesium.
 5. A process as claimed in claim 4 whereby the slag used in the binder has been allowed to cool naturally in air from the pouring temperature.
 6. A process as claimed in claim 4 in which the slag used in the binder is in finely divided form.
 7. A process as claimed in claim 1 in which the agglomerates are dimensioned to be within the range of 5-40 mm.
 8. A process as claimed in claim 7 in which the agglomerates are calcined by heating to a temperature within the range of 650*-800* C for from 30-90 minutes.
 9. A process as claimed in claim 7 in which the agglomerates are calcined at a temperature within the range of 900*-1,000* C for from 5-15 minutes. 